FIG. 1 shows a conventional arrangement of a cryostat including a cryogen vessel 12. A cooled superconducting magnet 10 is provided within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, which in this case is shown on the side of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements, by recondensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in FIG. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first stage temperature, typically in the region of 50-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K and may recondense the gas into liquid state.
A negative electrical connection 21a is usually provided to the magnet 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by either a conductor passing through the vent tube 20 or conduction through a component of turret 19.
For fixed current lead designs, a separate vent path (auxiliary vent) (not shown in FIG. 1) may be provided as a fail-safe vent in case of blockage of the vent tube 20.
The cryogen 15 is typically liquid helium at a temperature of about 4K, although other cryogens may be used such as liquid hydrogen, liquid neon or liquid nitrogen. At service intervals, it may be necessary to remove the refrigerator 17, and to open the vent tube 20. There is a risk that air could enter the cryogen vessel when the refrigerator is removed, or when the vent tube 20 is opened. Furthermore, experiments have shown that air continually diffuses into the cryogen vessel through the quench valve, and the vent valve, each conventionally provided on cryogen vessels, despite the cryogen vessel being held at positive pressure. Positive pressure means that the pressure of gas within the cryogen vessel is in excess of atmospheric pressure, so that any leak will primarily leak cryogen gas out of the cryogen vessel, rather than allow air to enter the cryogen vessel.
If air enters the cryogen vessel, it will freeze onto the coldest surfaces. With higher-boiling-point cryogens, such as nitrogen, only the water contained in air may be frozen. This may block the access between refrigerator and the cryogen vessel or, degrade the performance of the refrigerator, leading to a rise in temperature and pressure within the cryogen vessel, in turn leading to increased consumption of cryogen. The frozen deposit may also build up around the vent tube 20. The vent tube allows boiled-off cryogen gas to escape from the cryogen vessel, and is particularly important in the case of a magnet quench. During a magnet quench, a superconductive magnet suddenly becomes resistive, and loses all of its stored energy to the cryogen. This results in very rapid boil-off of cryogen. If the vent tube is constricted, or even blocked, then dangerously high pressure may build up within the cryogen vessel.
Removing a frost deposit from the inside of the cryogen vessel may require removing all of the cryogen and allowing the cryogen vessel and the magnet or other equipment within it to warm up—for example, to room temperature. This is a time consuming and costly process, as the removed cryogen will need to be replenished, and, in the case of a superconducting magnet, a shimming operation may need to be performed to correct any changes in magnetic field homogeneity which may have been brought about by the warming and re-cooling of the magnet. During this whole process, the apparatus cooled within the cryogen vessel, and the system of which it forms a part, is unusable. This may have consequential effects such as not being able to image patients, and their maladies remaining undiagnosed. Further cost implications are involved due to a very expensive imaging system being unavailable for a considerable period of time. It is therefore not practical to warm the cryogen vessels and their contents as a preventative service operation. However, by not performing such preventative measures, the danger of blockages and excessive cryogen pressures remains.
The present invention aims to provide apparatus and methods for detecting the presence of frost inside the cryogen vessel. The presence of a frost may then be signalled to a user or a service technician, and corrective action may be planned for a convenient time in order to remove the frost—for example by warming of the cryogen vessel.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.